Influenza viruses cause epidemic diseases every year and sometimes cause pandemic diseases taking millions of victims. Influenza infection is generally prevented by preventive inoculation with a vaccine. The vaccine is obtained by inactivating influenza virus particles or partially decomposed products thereof. According to preventive inoculation with influenza vaccines, it is possible to reduce the risk of serious complications such as pneumonia, hospitalization and death when a person is infected with influenza.
Currently, vaccines are produced by inoculating embryonated chicken eggs with influenza viruses. Therefore, a large amount of embryonated chicken eggs need to be prepared for vaccine production. In addition, since production takes about six months, mass production of unexpected vaccines is difficult. In addition, allergic reactions due to egg components need to be considered.
Further, when influenza viruses are grown in chicken eggs, mutations are introduced into the viruses' genes, and antigenicity of the viruses changes in many cases. Actually, in recent years, in H3N2 type and B-type vaccines, antigenicity of a selected vaccine strain has matched that of an epidemic strain. However, mutations have been introduced into vaccine virus genes at the production stage, and antigenicity of produced vaccines has changed from the original vaccine strains. Therefore, there are situations in which “the vaccine effect is low even when an epidemic strain has been predicted correctly” (refer to Non Patent Literature 1).